1. Field of the Invention
The present invention relates primarily to a cement admixture and a cement composition used in the civil engineering and construction fields.
2. Description of the Prior Art
Calcium aluminoferrite has been well known as a CaO—Al2O3—Fe2O3 based compound used for cement admixtures. The calcium aluminoferrite known so far in the art includes 4CaO.Al2O3.Fe2O3 (C4AF), 6CaO.2Al2O3.Fe2O3 (C6A2F), and 6CaO.Al2O3.2Fe2O3 (C6AF2).
These calcium aluminoferrites have a crystal structure of 2CaO.Fe2O3 (C2F) that is a sort of calcium ferrite. In short, they keep a C2F crystal structure although a large amount of Al2O3 is solid-solubilized in C2F, and take a variety of Al2O3/Fe2O3 molar ratios in terms of composition. The C2F crystal structure is an orthorhombic system where a=5.32 Å, b=14.48 Å and c=5.51 Å with a unit lattice volume of 424.95 Å3.
On the other hand, CaO.2Al2O3 (CA2) has been known as a sort of calcium aluminate. The crystal structure of CA2 is a monoclinic system where a=12.89 Å, b=8.88 Å and c=5.45 Å with a unit lattice volume of 596.41 Å3.
Thus, C2F and CA2 have quite different crystal structures, and never until now is any compound with a Fe2O3 component solid-solubilized in CA2 known in the art.
By the way, there has recently been mounting demand for improvements in the durability of concrete structures in the civil engineering and construction fields.
One factor of degradation of concrete structures is damage caused by salt where reinforcing bars (rebars) corrode visibly in the presence of chloride ions, and to hold back that damage, there is a method for imparting resistance to penetration of chloride ions to concrete structures.
In order to keep chloride ions from penetrating into a concrete structure thereby imparting resistance to penetration of chloride ions to it, there is a method of reducing water/cement ratios known in the art (see Non-Patent Publication 1). However, the method of reducing water/cement'ratios is not only detrimental to workability but may also provide no drastic measure.
There has also been a method of using a cement admixture composed mainly of CaO.2Al2O3 and gypsum and further containing an inorganic chloride for the purpose of imparting early strength to cement concrete and preventing corrosion of rebars (see Patent Publication 1).
In addition, there has been a method of using a cement admixture containing calcium aluminate having a CaO/Al2O3 molar ratio of 0.3 to 0.7 and a Blaine's specific surface area of 2,000 to 6,000 cm2/g thereby making sure good resistance to penetration of chloride ions and preventing temperature cracking of mass concrete (see Patent Publication 2). A problem with this cement admixture is, however, that a rapid-hardening feature appears in high-temperature environments with detrimental to the workability of cement concrete mixed with it. For instance, the rapid-hardening feature would not only appear in regions where high temperatures prevail and Chloride Attack and acidic degradation are accelerated such as Okinawa at home and Singapore abroad, but it has also emerged through numerous studies made by the inventors that the resistance to penetration of chloride ions is not fully exerted. Thus, there is still expectation of the development of a rustproofing technology that works more effectively in high-temperature environments where the diffusion speed of corrosive components is high and corrosion reactions are accelerated.
On the other hand, cement compositions mixed with finely pulverized, quenched, granulated blast-furnace slag and pozzolan substances have been known to make improvements in the resistance to penetration of chloride ions. Why they inhibit the penetration of chloride ions is that the Al component in the finely pulverized, quenched, granulated blast-furnace slag contributes to chemical fixation, or electrical absorption, of chloride ions. Referring here to reductions of calcium hydroxide in hardened cement, the pozzolan substances would appear to reduce voids of a few tens to a few hundred of micrometers that are otherwise formed in the case where calcium hydroxide is leached out in seawater. However, the reactions of finely pulverized, quenched, granulated blast-furnace slag and pozzolan substances tend to take place over an extended period of time, impeding the development of initial strength, and giving rise to a problem that as they are immersed in seawater in an early material age, it causes the resistance to penetration of chloride ions to go down, resulting in degradation of concrete. In order to improve durability, viz., seawater resistance, it is thus required to accelerate reactions in hardened cement, thereby reducing penetration of chloride ions under the action of seawater from an early material age.
On the other hand, there has also been a method for adding nitrites or the like proposed for the purpose of prevention rusting of rebars (see Patent Publications 3 and 4). However, the nitrites have been found to have no effect on acid resistance at all.